1. Volume 123, Issue 1, Pages 49-63 (October 2005)
Regulation of p53 Translation and Induction after DNA Damage by Ribosomal Protein L26 and Nucleolin 
Masatoshi Takagi, Michael J. Absalon, Kevin G. McLure, Michael B. Kastan 
Cell 
Volume 123, Issue 1, Pages (October 2005)
DOI: /j.cell
Copyright © 2005 Elsevier Inc. Terms and Conditions

2. Figure 1 Translational Control of p53 Protein Levels
(A) Cycloheximide blocks the induction of p53 after irradiation. p53 and actin protein levels in MCF7 cells treated with and without 10 mM cycloheximide (CHX) 10 min before 5 Gy irradiation (IR) were assessed by immunoblot. Cells were unirradiated (−) or harvested 10 or 20 min after IR.
(B) Translation of p53 is increased by irradiation. p53 protein was immunoprecipitated from MCF7 cells that had been labeled for 5 min with [35S]methionine 30 min after exposure to 0 or 10 Gy IR and was assessed by autoradiography. Cells had been pretreated with 50 μM MG132 and immunoblotting (WB) showed equivalent amounts of p53 in the immunoprecipitate (middle panel). Analysis of whole-cell extracts (WCE) showed equal amounts of [35S]methionine incorporated into the irradiated and unirradiated cells (bottom panel).
(C) The 5′UTR p53 mRNA affects the level of p53 protein. H1299 cells were transfected with p53 mRNA having 5′UTRs of various lengths, a full-length coding sequence, a complete 3′UTR, and poly(A). Firefly luciferase mRNA was an internal control. The amounts of p53, firefly luciferase, and actin present 24 hr after transfection were assessed by immunoblot.
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3. Figure 2 Identification of p53 5′UTR-Specific Binding Proteins
5 × 105 clones were screened to yield 128 primary HIS3-positive clones; 97 were positive for LacZ, 33 were selected by 5-FOA sensitivity, and 10 contained open reading frames.
(A) Binding of proteins expressed by each pACT2 clone to the p53 5′UTR and 3′UTR. Additional information on the binding proteins is described in Table S1.
(B) Candidate clones were transfected into MCF7 cells, and their effects on p53 and actin expression were compared by immunoblotting 24 hr after transfection.
(C) RNA-EMSA. 32P-labeled 5′UTR was incubated with His-control protein (−,−) or His-RPL26 (−,+); supershift was assessed by addition of anti-penta His antibody (+,+).
(D) RNA pull-down assay. Whole-cell extracts (WCE) prepared from MCF7 cells 1 hr after 0 (−) or 10 (+) Gy IR were mixed with biotinylated p53 5′UTR or 3′UTR. Input and bound fractions were analyzed by immunoblotting.
(E) RPL26 binds to p53 5′UTR in cells. Control IgG or anti-RPL26 (N-terminal and C-terminal mixture) was used to immunoprecipitate lysates from MCF7 cells 30 min after 0 or 10 Gy IR, and bound RNA was amplified by PCR of the 5′UTR of p53. PCR products were visualized by ethidium bromide staining. SM: size marker.
(F) Identification of p53 5′UTR binding proteins by RNA pull-down. Cytoplasmic extracts from MCF7 cells 1 hr after 0 or 10 Gy IR were incubated with biotinylated 3′ or 5′UTR of p53 and precipitated with streptavidin beads. Gel was stained by cypro ruby. Several proteins identified by mass spectrometric analysis are shown by labels. SM: size marker. Information on binding proteins is also shown in Table S2.
(G) IP-RT-PCR for p53 5′UTR as described in (E), using anti-nucleolin antibody or control mouse IgG.
Cell  , 49-63DOI: ( /j.cell )
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4. Figure 3
RPL26 Overexpression Increases p53 Protein Translation, Levels, and Function
(A) Levels of various proteins assessed by immunoblot 1 hr after 0 or 5 Gy IR in MCF7 cells transfected with increasing amounts of GFP-RPL26.
(B) Levels of various proteins assessed by immunoblot at various times after 0 (−) or 5 Gy IR in MCF7 cells transfected with a GFP-mock or a GFP-RPL26 vector.
(C) Reporter assay of p53 transcriptional activity using a generic p53 consensus target sequence or a p21 promoter sequence in cells cotransfected with a GFP-mock or a GFP-RPL26 vector. Firefly luciferase activity was evaluated 3 hr after exposure of cells to 0 (−) or 5 Gy IR relative to an internal Renilla luciferase control. Error bars show SEM.
(D) RPL26 stimulates p53 expression by increasing translation. Pulse-labeled p53 was immunoprecipitated from MCF7 cells transfected with increasing amounts of GFP-RPL26 30 min after 0 or 5 Gy IR and assessed by autoradiography (second panel) and immunoblot (third panel). Total amount of GFP-RPL26 was assessed by immunoblot (top panel), and total [35S]methionine incorporation into cellular proteins was assessed by autoradiography (bottom panel).
(E) As in (D), but using SW480 cells.
(F) GFP-RPL26 overexpression specifically increases translation of p53 mRNA. [35S]methionine pulse-labeled p53, ERK3, and c-Myc were immunoprecipitated from MCF7 cells transfected with GFP-mock or GFP-RPL26 and assessed by autoradiography and immunoblot.
(G) RPL26 alters the polysome distribution of p53 mRNA. RNA from MCF7 cells transfected with a GFP-mock or GFP-RPL26 vector was fractionated by sucrose-gradient centrifugation and analyzed by Northern blot (1, lightest fraction; 9, heaviest fraction). Fractionated ribosomal RNA was also resolved by agarose gel electrophoresis and visualized with ethidium bromide (bottom panel).
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5. Figure 4
Reduction of RPL26 Levels Reduces p53 Protein Translation and Induction
(A) Immunoblot of various proteins in MCF7 cells transfected with a control siRNA or siRNA against RPL26 after 0 (−) or 5 Gy IR.
(B) An siRNA-resistant RPL26 vector re-establishes p53 induction in siRNA-treated cells. Top panel shows the schema of the siRNA and mutant RPL26 vector. MCF7 cells were transfected with a control siRNA or siRNA against RPL26 and then transfected with a GFP-mock vector (mock) or the GFP-RPL26 deletion mutant plasmid (RPL26mt) 24 hr after siRNA transfection. Twenty-four hours after plasmid transfection, cells were irradiated and were harvested 3 hr later, and protein levels were assessed by immunoblot.
(C) p53 translation is complemented by the siRNA-resistant mutant RPL26 vector. A GFP-mock vector (mock) or the GFP-RPL26 deletion mutant plasmid (RPL26mt) was transfected into MCF7 cells previously transfected with RPL26 siRNA as described in (B). Cells were exposed to 0 or 5 Gy IR and pulse-labeled (5 min) with [35S]methionine, and newly synthesized p53 was assessed by autoradiography. Total levels of immunoprecipitated p53 and [35S]methionine incorporation into total cellular proteins are shown.
(D) GFP-RPL26 knockdown specifically decreases translation of p53 mRNA. [35S]methionine pulse-labeled p53, ERK3, and c-Myc were immunoprecipitated from MCF7 cells transfected with control siRNA or RPL26 siRNA and assessed by autoradiography and immunoblot.
(E) RPL26 siRNA reduces translocation of p53 mRNA to heavier polysomes after IR. MCF7 cells transfected with a control siRNA (upper three panels) or siRNA against RPL26 (lower three panels) were exposed to 0 or 5 Gy IR, and RNA extracted 30 min later was fractionated by sucrose-gradient centrifugation. Fractions were analyzed as described in Figure 3.
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6. Figure 5 Mapping RPL26 and p53 5′UTR Interaction Domains
(A) Capped p53 mRNA containing variable 5′UTR sequences was cotransfected with firefly luciferase mRNA and either a GFP-RPL26 expression vector or a GFP-mock expression vector into H1299 cells, and protein levels were assessed by immunoblot. Equivalence of transfection efficiency, mRNA content, and expression is indicated by the similarity of the firefly luciferase levels.
(B) Effects of RPL26 on p53 translation were assessed in rabbit reticulocyte lysates containing p53 cDNAs with variable 5′UTR sequences, firefly luciferase cDNA, and either His-RPL26 or His-control cDNA. Expression was assessed by immunoblot.
(C) Various RPL26 deletion mutants were transfected into MCF7 cells, and endogenous p53 levels were assessed by immunoblot.
(D) Schematic summary of localization, 5′UTR binding, and p53 induction of RPL26 deletion mutants. Nclo., nucleoli; Ncl., nucleoplasm; Cyt., cytoplasm.
(E) Binding of wild-type and mutant constructs of RPL26 to the p53 5′UTR, assessed by IP-RT-PCR using anti-GFP antibody. PCR products were visualized by ethidium bromide staining.
Cell  , 49-63DOI: ( /j.cell )
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7. Figure 6 Cellular Consequences of RPL26 Overexpression or Knockdown
(A) Apoptosis of BaF3 cells induced by 5 Gy IR was measured in GFP-mock- or GFP-RPL26-overexpressing cells, which were cotransfected with mock empty vector, E6, or dominant-negative p53. The percentage of apoptotic cells was determined by annexin V staining 3 hr after IR. Cells were grown in the presence (open bars) or absence (filled bars) of IL-3. Error bars show SEM.
(B) Apoptosis of BaF3 cells induced by 5 Gy IR was measured in control-siRNA-transfected and RPL26-siRNA-transfected cells, and the percentage of apoptotic cells was determined as above.
(C) Induction of apoptosis of HCT116 parental or HCT116 p53 null cells at various times by 300 μM 5-fluorouracil treatment was measured by annexin V staining.
(D) An experiment similar to (C) was performed in control-siRNA- or RPL26-siRNA-transfected cells.
(E) MCF7 cells overexpressing GFP-mock or GFP-RPL26 were cotransfected with mock, E6, or dominant-negative p53, and cell-cycle distribution was determined by BrdU pulse-label 24 hr after transfection. The G1/S ratio ± SEM obtained from three independent experiments is shown. Open bars, mock transfectants; filled bars, RPL26 transfectants.
(F) Colony-survival assay of GFP-mock- and GFP-RPL26-transfected arf−/−p53+/+ mouse embryonic fibroblasts (MEFs) and arf−/−p53−/− MEFs; crystal-violet-stained cells are shown.
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8. Figure 7 Modulation of p53 by Nucleolin Manipulation
(A) Levels of p53 and p53 targets were assessed by immunoblot at various times after 5 Gy IR in MCF7 cells stably transfected with a GFP-mock vector and a GFP-nucleolin vector.
(B) Levels of [35S]methionine-labeled p53 in the same MCF7 cells 30 min after 0 (−) or 5 (+) Gy IR.
(C) shRNA against nucleolin enhances p53 expression. MCF7 cells transfected with a control shRNA or shRNA against nucleolin (NCL) exposed to 0 or 5 Gy were harvested 1 hr after IR and analyzed by immunoblot.
(D) As in (C), levels of [35S]methionine-labeled p53 in the same MCF7 cells 30 min after 0 or 5 Gy IR.
(E) Nucleolin overexpression induces REF transformation. Upper panel shows the morphological change of REFs transformed with oncogenic Ras and GFP-mock, mutant p53 (R175H), or GFP-nucleolin; lower panel shows colony-formation activity in soft agar plate.
(F) Competition between RPL26 and nucleolin in modulating p53 protein levels. Levels of various proteins were assessed by immunoblot in MCF7 cells that had been transiently transfected with nucleolin (NCL) and varying amounts of GFP-RPL26. Extracts were made 30 min after exposure to 0 or 5 Gy IR.
(G) The effects of nucleolin and varying amounts of RPL26 on p53 translation in MCF7 cells were assessed as described above.
Cell  , 49-63DOI: ( /j.cell )
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